1. Field of the Invention
This invention relates to useful stabilizer compositions and to improved propylene polymer compositions stabilized therewith. The stabilized propylene homopolymers and copolymers have improved resistance to discoloration upon processing.
2. Description of the Prior Art
The sensitivity of propylene polymers to oxidative attack and the need to stabilize these polymers against degradation which can occur during processing and over the life of the polymer is well known. For example, when an unstabilized or insufficiently stabilized polypropylene or other high propylene content copolymer is processed, a significant increase in melt flow rate due to degradation, i.e., chain scission, of the polymer chain is observed. Processing stabilizers, such as sterically hindered phenols, phosphites, phosphonites and combinations thereof, are added to control this rise in melt flow rate.
Additionally, primary antioxidants or stabilizers are incorporated to impart long term stability to the manufactured product. The amount and type of stabilizer(s) used for this purpose depends on the conditions to which the product will be exposed and the desired service life. Sterically hindered phenols of medium to high molecular weight are used almost exclusively for this purpose, often in conjunction with synergists, such as thioethers. Phosphites added to provide stabilization during processing may also function as the primary antioxidant, i.e., impart long term post-processing stability, if utilized at sufficiently high levels.
The complete stabilizer package will also most often include one or more additional additives, such as ultraviolet absorbers, metal deactivators, acid neutralizers and catalyst deactivators. The addition of such compounds protect the propylene polymer against specific types of degradation and further improve retention of melt viscosity, color and physical properties. Calcium stearate, zinc stearate, lactic acid salts, i.e., lactates and lactylates, zinc oxide and synthetic hydrotalcites, are examples of additives commonly included in propylene polymer stabilizer packages.
Recently, biological antioxidants have also been suggested as useful polypropylene stabilizers. Vitamin E (dl-.alpha.-tocopherol) is commercially synthesized and has been suggested as a polypropylene stabilizer by itself and in combination with other hindered phenol primary antioxidants and/or secondary antioxidants.
By way of illustration, stabilizer combinations useful for propylene polymers are discussed in U.S. Pat. Nos. 3,219,622; 4,366,280; 4,912,156; 5,063,264 and 5,227,418. The latter reference, for example, utilizes a combination of tetrakis (methylene 3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate) methane; bis (2,4-di-t-butylphenyl)pentaerythritol diphosphite and calcium lactate to stabilize stereoregular branched mono-1-olefin polymers, such as polypropylene.
Whereas effective protection against oxidative degradation of propylene polymers can be achieved with a variety of stabilizer packages, there is often a noticeable deterioration of color during processing. This discoloration, i.e., yellowing, even when slight, is undesirable in all instances and cannot be tolerated for certain critical use applications. The degree of discoloration of propylene polymers is generally determined by measuring the yellowiness index (YI) in accordance with ASTM D 1925. Stabilizer combinations which exhibit a reduced tendency to discolor propylene polymers are highly desirable and always in demand.